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J. Bacteriol. doi:10.1128/JB.00897-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Oxygen reactivity of both respiratory oxidases in Campylobacter jejuni: the "cydAB" genes encode a cyanide-resistant, low-affinity oxidase that is not of the cytochrome bd-type

Department of Molecular Biology and Biotechnology, University of Sheffield, Western Bank, Sheffield, S10 2TN, UK, and School of Biomedical and Molecular Sciences, University of Surrey, Guildford, Surrey, GU2 7XH, UK

^* To whom correspondence should be addressed. Email: r.poole{at}sheffield.ac.uk.

	Abstract

The microaerophilic bacterium Campylobacter jejuni is a significant food-borne pathogen, predicted to possess two terminal respiratory oxidases with unknown properties. Inspection of the genome reveals an operon (cydAB) apparently encoding a cytochrome bd-like oxidase homologous to oxidases in Escherichia coli and Azotobacter vinelandii. However, C. jejuni cells lacked all spectral signals characteristic of the high-spin hemes b and d of these oxidases. Mutation of the cydAB operon of C. jejuni was without significant effect on growth, but the mutation reduced formate respiration and viability of cells cultured in 5 % oxygen. Since cyanide-resistance of respiration was diminished in the mutant, we propose that C. jejuni CydAB be renamed CioAB (cyanide-insensitive oxidase) as in Pseudomonas aeruginosa. We measured the oxygen affinity of each oxidase using a highly sensitive assay that exploits globin deoxygenation during respiration-catalyzed oxygen uptake. The CioAB-type oxidase exhibited a relatively low affinity for oxygen (K_m = 0.8 µM) and a V_max of >20 nmol/mg/s. Expression of cioAB was elevated 5-fold in cells grown at higher rates of oxygen provision. The alternative ccoNOQP-encoded cyanide-sensitive oxidase, expected to encode a cytochrome cb'-type enzyme, plays a major role in the microaerobic respiration of C. jejuni since it appeared to be essential for viability and exhibited a much higher oxygen affinity, with a K_m value of 40 nM and a V_max of 6-9 nmol/mg/s. Low-temperature photodissociation spectrophotometry revealed that neither oxidase has ligand-binding activity typical of the heme-copper oxidase family. The data are consistent with cytochrome oxidation during photolysis at low temperature.

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